Because of their high hardness, high fracture toughness, chemical stability, and high temperature resistance, abrasive grains on the basis of aluminum oxide are industrially used in large quantities for the production of abrasive articles. In addition to fused aluminum oxide which is cost-efficiently produced in electric arc furnaces and which, on a quantity basis, represents the largest percentage of abrasive grains for the production of abrasive articles, sintered abrasive grains manufactured by ceramic or chemical processes are often used for special applications. The advantage of sintered abrasive grains for special grinding operations results from their microcrystalline structure and/or their geometrically specified shape.
Generally, the grinding performance of an abrasive grain or an abrasive article is determined by the so called g-ratio which is calculated as the ratio of stock removal to abrasive wear. High hardness of the abrasive grain effects high stock removal rates and consequently a high g-ratio. Simultaneously, high fracture toughness of the abrasive grain yields to low abrasive wear and as well to a high g-ratio. Thus, in the ideal case, hardness and fracture toughness of the abrasive grain should be as high as possible for obtaining a high g-ratio.
However, due to the different physical properties of the plurality of materials which are to be processed, such as, e.g., wood, steel, stainless steel, plastics, stone, ceramics, among others, the abrasive grain has to meet different requirements with regard to its hardness and fracture toughness, respectively, with regard to the ratio of hardness to fracture toughness.
This ratio can be influenced for instance by modifications of the chemical composition. However, the grinding performance of an abrasive grain is not only characterized by its chemical composition, but furthermore by its crystalline structure, its density, and not least by its geometrical shape. Moreover, the decision whether an abrasive grain is used for specific grinding operations is not only influenced by its potential grinding performance, in many cases the grain production costs depending substantially on the raw material costs and the process conditions play an important role.
A sintered rod shaped abrasive grain is described in U.S. Pat. No. 3,387,957, which grain is produced by extruding a mixture of finely grounded bauxite, water and a binder. Such obtained elongated extruded green bodies having a uniform circular cross section are subsequently cut into grains of a definite length, which grains are subsequently sintered at a temperature range between 1300° C. and 1600° C. Such rod shaped sintered abrasive grains are preferably used in resin bonded grinding wheels for heavy duty grinding operations, such as snagging of billets of stainless and high alloy steels.
Due to the raw materials which are used, beside aluminum oxide, the resulting abrasive grains comprise silicon oxide, iron oxide, titanium oxide as well as small amounts of other oxides, such as calcium oxide and magnesium oxide. Some of those oxides form separate mineralogical phases in combination with aluminum oxide, such as mullite (3 Al2O3*2 SiO2) or tialite (Al2TiO5), which phases have a lower hardness than aluminum oxide (corundum) effecting that the corresponding abrasive grains are also less hard, however, due to segregations or inclusions of the different phases, the abrasive grains exhibit often a higher fracture toughness which could be advantageous for specific grinding applications. In this case, the possible fields of application of the corresponding abrasive grains are additionally extended, because bauxite is used as low-priced raw material. However, an adverse effect of using natural occurring bauxite is caused by possible variations of the chemical composition which could have a disadvantage effect on the product quality.
A sintered abrasive grain having a density of more than 3.75 g/cm3, a Knoop hardness of more than 1.900 kg/mm2, an aluminum oxide content of more than 98% by weight, and a grain structure configured by a mixture of coarse and fine crystal particles, wherein the coarse crystal have an average particle size in a range of 3 to 10 μm and the small crystal particles have an average particle size of less than 2 μm, is disclosed in U.S. Pat. No. 4,252,544. The abrasive grain is produced by mixing coarse electrofused or high temperature calcined alumina powder having a particle size in the range of 3 to 10 μm and fine powder having particle sizes smaller than 1 μm in the presence of water and a binder; extruding the mixture; drying the extruded material, while cutting it to a determined length; and sintering the cut and dried pieces at a temperature between 1550° C. and 1650° C. Abrasive grains such obtained have a higher hardness compared with abrasive grains on the basis of bauxite, resulting in a high stock removal rate, whereas the wear of the abrasive article simultaneously increases. The increased raw material costs have an adverse effect and make the production more expensive.
U.S. Pat. No. 2,360,841 describes sintered abrasive grains consisting of aluminum oxide crystals containing titanium oxide and iron oxide in solid solution. One embodiment is described comprising additionally zirconium oxide as crystal growth inhibitor.
A sintered abrasive grain useful for heavy duty grinding or snagging operations comprising 30 to 70 percent by weight aluminum oxide, 15 to 60 percent by weight zirconium oxide, and 5 to 15 percent by weight one or more oxides selected from the group consisting of iron oxide, titanium oxide, manganese oxide and silicon oxide, is described in U.S. Pat. No. 3,454,385.
Subject matter of U.S. Pat. No. 3,481,723 are cylindrical abrasive grains consisting essentially of an abrasive material selected from the group consisting of aluminum oxide, zirconium oxide, zirconium silicate, silicon carbide, titanium oxide, manganese oxide, bentonite, silicon and mixtures thereof.
EP 2 636 655 A1 describes abrasive grains on the basis of an alumina sintered compact having a high hardness and a high fracture toughness. For the production of the abrasive grain, alumina powder and ilmenite powder are added to an aqueous medium containing a binder. Subsequently, the mixture is homogenized, shaped to have a desired form, and sintered to obtain sintered shaped abrasive grains having high hardness and excellent fracture toughness. The fracture toughness of the abrasive grains is increased by the formation of FeTiAlO5 crystals in the grain boundary of alumina crystals.
All the above mentioned shaped abrasive grains are used for heavy grinding operations whereby material is to be removed as fast and as much as possible. Snagging or rough grinding of slabs and billets in foundry and steel production industry are examples for such applications, whereby for example highly densified nearly pore free hot pressed resin bonded wheels are used. As mentioned above, the suitability of a compact solid body for the use as abrasive grain substantially depends on its hardness and its fracture toughness. However, also other parameters play a role, e.g. such as bonding forces to hold the abrasive grain in the binding matrix of the grinding wheel and such forces (pressure forces) that are acting on the abrasive grain externally during the grinding process. Considering the wear mechanism of abrasive grains, the abrasive grain firstly wears itself off and blunts until parts of the grain are broken and new cutting edges are formed due to external pressure. In case of very high fracture toughness of the abrasive grain, there is the possibility of a reverse effect in which the whole grain quarries out of the bonding and is therefore lost for the grinding process. Consistently, different materials are treated under different circumstances; therefore, it is necessary to firstly adapt the abrasive grain to the material to be processed as well as to the conditions of grinding and abrasive forces that may occur and to secondly optimize the abrasive grain particularly regarding the interaction between hardness and fracture toughness.
Hence, the task of the present invention is to provide abrasive grains featuring the highest possible hardness and fracture toughness for specific uses. It is another task of this invention to provide a way of producing such optimized abrasive grains in accordance with an acceptable cost/performance ratio.